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Medical Manufacturing: Living in the Materials World

A look at the qualities and challenges of some medical metals with ATW Companies' Tracy MacNeal

By Michael C. AndersonSenior Editor

The North American medical device industry has been growing at a healthy clip for years, but is facing more pressure to create high-quality products at lower costs than ever before. Health care reform and FDA stringency in the US along with the need to compete for market share in the developing world are the pincers squeezing medical device OEMs to find ways to retain quality, increase innovation, and reduce product cost all at the same time. For any manufacturer in the industry, an important area where these concerns come together is that of materials choice.

The range of FDA-approved materials available for medical manufacturers is varied and growing. Tracy MacNeal, chief strategy officer at ATW Companies (Warwick, RI), a provider of highly engineered metal solutions to the metal component marketplace, expects growth in the medical industry, which currently accounts for 40% of its manufacturing.

MacNeal’s entire career has been in FDA-regulated industries; she kindly walked ME Media through a virtual bazaar of medical materials and discussed their strengths and challenges.

Implants: Biocompatibility and Wear Issues

Implants, orthopedic and otherwise, are all FDA Class Two and Class Three devices, with stringent requirements, the foremost of which is biocompatibility, MacNeal said. “Of the materials that are favored from a biocompatibility standpoint in metals, titanium would be the number one choice for implants—it’s basically inert in the body. There are also some alloys of stainless steel—people talk about ‘surgical stainless steel’ and those two would be the two big ones.”

But as manufacturers in other industries know, titanium has its challenges: “Titanium is difficult to work with because it does catch fire. When you’re machining it, you really have to control your feeds and speeds. Its ratio of hardness to brittleness is not great, and it doesn’t have good wear properties—it abrades. In an articulating joint like a knee or hip, you can’t have metal-on-metal there, it’s much too soft.” Cobalt chrome, another popular medical metal, has been used as a wear surface in orthopedic implants, but, as has been widely reported, it’s under fire right now: “People who have cobalt chrome metal-on-metal interfaces in their orthopedic joints get wear debris resulting in much higher than average levels of chromium ions in their body,” MacNeal noted.

“Those higher levels weren’t planned for and weren’t in the original filing data, so even though they haven’t been linked to any health problems, they’re an unexpected outcome, and the FDA is asking questions. So orthopedics companies are trying to get away from cobalt chrome for such applications.

“Instead of a metal-on-metal wear surface, companies typically will have a HDPE—high-density polyethylene wear surface, which simulates cartilage. In an actual hip joint, the bone is covered with cartilage, which when lubricated with synovial fluid is essentially friction-free. Inside the orthopedic joint, a biocompatible metal is coated with HDPE to mimic the cartilage role. But HDPE too can abrade, and in this case, the wear debris—inert polyethylene particles—builds up behind the metal, and as the body attempts to clean up these wear particles it can trigger an autoimmune reaction which causes resorption of bone tissue—a condition called osteolysis. The bone pulls away from the metal joint that had been screwed into it, and the joint can start to become loose. That’s usually why some patients need revision surgery—and why people who are, say, 65 years old may elect to put off having replacement surgery, in order to not need to replace the joint at age 75.

“So the hunt has been on for a better wear surface. Enter ceramics. Ceramics are super-hard and are great wear surfaces. They don’t abrade so you don’t have the wear debris issues. There have been two issues with ceramics, however. The first is that if the clearance isn’t completely and totally perfect, you end up with ceramic squeaking against ceramic: As people were taking steps, their joints were literally squeaking—loudly!—and these are permanent implants, so there’s no easy way to minimize the sound. The other challenge with ceramics is that they’re comparatively brittle—if they receive the wrong impact, they break, creating a problem much worse than noisy joints.”

While the squeaking is a quality of life problem, it is not much of a wear problem. Ceramics are essentially self-lubricating. But there are also serious machining issues with ceramics. They are extremely hard, so shaping them is a problem—especially when you need such a perfect fit.

Nitinol: Thanks for the Memory

A material that is growing in popularity for certain applications is the titanium/nickel alloy nitinol, which has shape-memory capabilities that make it exceptional, MacNeal says, and there are players in the industry that specialize in making devices that take advantage of that ability.

“A common example would be nitinol stents,” she notes, “which can be manufactured in a shape needed to rebuild a blood vessel, then collapsed to a much narrower diameter for easier insertion into the vessel, and finally allowed to resume its ‘remembered’ original shape as a scaffold to support the blood vessel.”

Shape-memory nitinol is also used for filters deployed in the aorta—if a blood clot gets through the aorta into the heart, it can mean instant death for the patient. MacNeal is impressed with the nitinol-based solution. “These filters are amazing—shape memory allows them to be inserted in a compact form, but when they deploy, they look like fishing lures, with tiny prickers or barbs that extend out to catch clots before they can enter the heart.”

Nitinol is also a metal popular in angioplasty applications: “The cardiac sector of the medical device industry is huge—second in size only to orthopedics. Cardiac applications call for companies that are good with wires; pulling wires and forming wires ... nitinol use is at the forefront of much of that.”

Competing in the Disposables Sector

Plastics are playing an important role in the disposables market. For hospitals, one-use products such as syringes and IV bags are easier to deal with, in terms of quality and sterilization concerns, said MacNeal.

“If it’s a single-use throwaway, you know that unless there was a problem at the manufacturer’s sterilization facility, the product can be trusted. On the other hand, disposables are costly. And in particular, how are you going to enter the emerging markets where a lot of industry growth is happening, when these products are so expensive? While a single disposable product itself may be cheap, the number needed can make them an expensive choice.

“Some hospitals are looking at reclaim and reuse, and considering investing in metal versions of the products and sterilization processes and reuse instead of disposables.

Our company is working with an OEM to develop a disposable metal suturing device. In this case metal is desirable for its strength. There’s a cantilevering action involved for which plastic just isn’t strong enough in the size the company is looking at. But machining the piece from metal would be prohibitively expensive, so they’re looking at manufacturing the device through the use of metal injection molding—MIM, one of our company’s primary technologies.

MIM: Sinter of Excellence

“With MIM, you start with powdered metal with a consistency similar to flour, and mix it with a binder—usually a polymer—and heat it so that the binder can flow but the metal itself hasn’t melted. The mixture is then injected into a mold, resulting in what we term a green part: the mixture has been shaped by the mold but the metal content is still solid—it’s just held in place by the binder.

The green part is then put into a sintering furnace and the binder bakes off, while the metal particles are heated just enough to touch and adhere directly to each other. If the furnace gets too hot, the metal would melt and the part would lose its shape, but with precise heating, the particles touch and sinter together to create a net-shape part.

“There’s a shrinkage factor because of the binder removal—the part will be 15–25% smaller than when it went into the furnace. Maximizing the metal-to-binder ratio, controlling flow and finessing the amount of shrinkage are areas that call for expertise. The benefits over machining a part include saving time and saving raw material because you’re creating a net-shape part. In the sweet-spot of using MIM, the process is 50% of the cost of creating the same part through machining raw metal.

“MIM is playing a role in keeping some medical devices relevant in the current trend toward disposables. Our MIM technology is a runaway train right now, in terms of demand, because the whole industry is looking to MIM to try to wring cost out of the system. Both health care reform in the US and Europe and the need to compete on cost in the developing world are forcing the OEMs to find ways to dramatically reduce costs. They know they’ve got to start looking at more innovative technologies, and MIM is on the list.”

The Advance of Additive

“Additive manufacturing is another innovative technology that has been interesting to see being developed. They’ve come a long way—I think they initially had strength issues—a laser-sintered part would be less strong than its machined counterpart—but I understand that they’ve come a long way. A process such as direct metal sintering could be considered a rival to MIM if it was being used for product runs of say 5000–15,000/year range.

"I see additive right now as being used for lower-volume, mass-customization niches; it offers the flexibility of making 3000 of something this year and 2000 of something different the next year. Right now, we’re a high-volume player—we’re involved with major companies needing higher volumes.” ME

This article was first published in the May 2013 edition of Manufacturing Engineering magazine. Click here for PDF.